"The team has a long-term plan for the future and I'm very much looking forward to playing a key role in its journey back to success".
"And this year, while still driving at my best, is how I want to remember you".
McLaren Racing confirmed Tuesday the Spaniard will not return to F1 next year for an 18th season, and Alonso said F1's leadership group tried to persuade him to stay with the series.
'Fernando is of course one of them, so it's particularly special that I'll be taking his seat as part of the next generation of Spanish racing drivers behind the wheel of a McLaren.
Only last week, Red Bull ruled out signing Alonso as a replacement for the Renault-bound Daniel Ricciardo because as team boss Christian Horner said: "He's tended to cause a bit of chaos wherever he's gone".
Alonso, into his 17th F1 season and his fifth with McLaren, is ninth in the drivers' championship standings.
After spending a single turbulent season at McLaren in 2007 after leaving Renault, Alonso returned to the Enstone outfit for two more years before switching to Ferrari where he twice narrowly missed out on winning the drivers' championship. I have built so many great relationships with many fantastic people at McLaren, and they have given me the opportunity to broaden my horizons and race in other categories.
Sainz, who is racing for Renault this season, said it made a dream move to McLaren even better. He has not finished on the podium since rejoining McLaren. He could join Toro Rosso in place of the promoted Gasly alongside Brendon Hartley.
Verstappen was promoted to Red Bull early into 2016 but Sainz, adamant that he deserved the call-up, continued to enhance his reputation as one of the sport's hottest young talents.
Ricciardo's move to Renault pairs him up with Nico Hulkenberg, who is on a multi-year deal, and leaves Sainz without a seat for 2019.
The closest he has been to the podium was a fourth place in Singapore a year ago, but he is widely seen as a promising talent, when put in the right team with the right technology.
Former F1 driver turned Sky Sports pundit Martin Brundle also referenced Alonso's combative nature and "unfulfilled potential". Get Sky Sports F1 - the home of every race weekend LIVE in 2018.